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Diaphragm Dishing Repair

Tuesday, February 24, 2015 • Kelly Plunkett

Mechanical Dynamics and Analysis inspected and repaired a stage 15 dished diaphragm. The unit was an upgraded 297.5 MW D6 frame unit. The stage 15 diaphragm was found to be dished downstream by approximately 0.215”.

MD&A’s Turbine-Generator Repair Facility performed a dishing repair to the stage 15 diaphragm by cutting out the old weld, re-positioning, re-welding, stress relief, and final machining. Incoming inspections and evaluation consisted of blast cleaning, NDE, visual inspections, and dimensional checks. The stage 15 drop checks were taken as a baseline for the as received diaphragm. Based on the opening clearance measurements, it was found that the stage 15 diaphragm was dished downstream by approximately 0.215”.

Due to the unacceptable amount of dishing found on the stage 15 diaphragm, the stage 15 diaphragm was repaired utilizing a welding and post weld heat treating process.

The repair process included machining to remove the spacer bands to inner and out ring welds, re-positioning the diaphragm back into the correct axial position, restoring the welds to provide an 100% full penetration weld, and then post weld heat treatment.

After post weld heat treatment, the diaphragm halves were disassembled from the jacking and backing plates, blast cleaned, and dimensional checks were performed to verify and ensure the diaphragm halves inner web rings were re-positioned back into the correct axial position.

The dimensional checks confirmed that the diaphragm halves inner web rings were positioned back upstream by between 0.240” and 0.260”. Once confirmed, each diaphragm half HJ was setup on a horizontal boring mill to machine and restore the HJ surface as well as the dowel holes and keyways. The diaphragm halves were assembled and the assembled diaphragm was setup on a vertical boring mill to final turn machine. Once all machining was completed, a final NDE was performed on all welded and machined surfaces. Lastly, the stage 15 diaphragm was assembled to the stage 16 diaphragm.